[Registry: Unused file extensions]

From edpb, many thanks to all who replied to my question about possible CCleaner actions when cleaning items from its "unused file extension" list. There are many such items, and many are not really unused, so until I get more experience with this I will just uncheck all of them and let them be.

[Registry: Unused file extensions]

In Registry, if I click Fix, what will happen to files with extensions that are on the unused extensions list? I have many active files with extensions on the unused extensions list.